Evaporative Deposition of Compounds
Evaporation processes for the deposition of compounds can be divided into two types:
1. Direct Evaporation in which the evaporant is the compound itself; and PA1 2. Reactive Evaporation and Activated Reactive Evaporation (ARE), in which the evaporant is a metal or a low-valence compound.
Direct Evaporation Processes
As a general case, when a common refractory compound is evaporated or sputtered the material is not transformed to the vapor state as compound molecules but as fragments of molecules. Many compounds including common refractory compounds, having this property, are said to vaporize incongruently. In the deposition of such vapors the fragments have to recombine, most probably on the substrate, to reconstitute the compound. This often leads to the deposition of sub-stoichiometric films.
Reactive Evaporation Processes
In reactive evaporation, in which a metal is evaporated in the presence of a reactive gas, a compound is formed by reaction of the evaporated metal species with molecules of the reactive gas. Though this technique has been extensively used to deposit a variety of films, it is generally observed that such films are sub-stoichiometric. It is also observed in some cases, especially in the synthesis of carbide films, that the deposition rate becomes a limiting factor governing the growth of the films. This limitation of deposition rate in the case of the reactive evaporation process is due to the reaction kinetics of the compound formation by this process. The presence of a "plasma" in the process of activated reactive evaporation, ARE, influences the reaction kinetics by providing the activation energy to the reactive species thereby synthesizing compounds films at considerably higher rates and lower temperatures.
Current Methods in Chemical Vapor Deposition
Chemical vapor deposition (CVD) is a process in which a surface is coated with vapors of volatile stable chemical compounds at a temperature below the melting point of the surface. One advantage of the process is the deposition of coatings varying in thickness, while main disadvantages are the necessity of heating the surface to be coated and the possibility of reactant-substrate interactions. In most cases, CVD reactions are activated thermally by use of a high substrate temperature. Another means of activation is obtained by electric-discharge plasma , i.e., plasma-assisted chemical vapor deposition (PACVD).
Problems Associated with the Use of Plasmas in Deposition Processes
In plasma-assisted processes, the plasma contains mixed reaction products including neutral particles, positive and negative ions, and electrons which may have adverse effects on the deposited material such as preferential sputtering. Any impurities carried into the plasma, from the constructional components of the plasma source, may be transferred to the deposited material. The introduction of such impurities creates problems, for example, in semiconductor device manufacturing and in the production of optical films. In addition the films deposited by ARE or PACVD are generally more complex than their analogues deposited by normal reactive evaporation or CVD.
Monitoring of a Surface Reaction by Work Function Measurements
The work function is a measure of the chemical potential and is a fundamental parameter of solid surfaces. Any changes in the surface state, such as chemical composition or geometric reconstruction, can be very precisely monitored through work function measurements. Thus far, no application of such a monitoring process has been used for the formation of molecules useful in deposition processes.